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In recent years, the electronic structures of organocuprates in general and the complex [Cu(CF3)4]− in particular have attracted significant interest. A possible key indicator in this context is the reactivity of these species. Nonetheless, this aspect has received only limited attention. Here, we systematically study the series of tetra‐alkyl cuprates [MenCu(CF3)4−n]− and their unimolecular reactivity in the gas phase, which includes concerted formal reductive eliminations as well as radical losses. Through computational studies, we characterize the electronic structures of the complexes and show how these are connected to their reactivity. We find that all [MenCu(CF3)4−n]− ions feature inverted ligand fields and that the distinct reactivity patterns of the individual complexes arise from the interplay of different effects. [ABSTRACT FROM AUTHOR]

In den letzten Jahren haben Organocuprate im Allgemeinen und der Komplex [Cu(CF3)4]− im Besonderen wegen ihrer elektronischen Strukturen erhebliches Interesse auf sich gezogen. Obwohl der Reaktivität dieser Spezies in diesem Zusammenhang eine Schlüsselrolle zukommen dürfte, fand dieser Aspekt bisher nur wenig Beachtung. Wir untersuchen hier systematisch die Reihe der Tetraalkylcuprate [MenCu(CF3)4−n]− und ihre Gasphasenreaktivität, die sowohl konzertierte reduktive Eliminierungen als auch Radikalverluste umfasst. Mit Hilfe quantenchemischer Rechnungen charakterisieren wir die elektronischen Strukturen der Komplexe und zeigen, wie sie mit der Reaktivität zusammenhängen. Wir finden, dass alle Ionen [MenCu(CF3)4−n]− invertierte Ligandenfelder aufweisen und dass sich die unterschiedlichen Reaktivitäten der individuellen Komplexe aus dem Zusammenspiel verschiedener Effekte ergeben. [ABSTRACT FROM AUTHOR]

The surprising differences between the experimental solid‐state and calculated gas‐phase structures of 5‐oxo‐1,3,2,4‐dithiadiazole (Roesky's ketone, 1) and 1‐oxo‐1,2,4,3,5‐trithiadiazole (Roesky's sulfoxide, 2), identified and studied in a series of papers published between 2004 and 2010 but then never satisfactorily explained, have been revisited, making use of the more advanced computational possibilities currently available. The previous calculations' considerable overestimations of the C−S and S−S bond lengths in 1 and 2, respectively, have been partly explained based on the results of periodic calculations and the application of Valence Bond (VB) Theory. In the case of 1, the crystal environment appears to stabilize a structure with a highly polarized C=O bond, which features a C−S bond with considerable double‐bond character – an effect which does not exist for the isolated molecule – explaining the much shorter bond in the solid state. For 2, a similar conclusion can be drawn for the S−S distance. For both compounds, though, packing effects are not the sole source of the differences: the inability of Density Functional Theory (DFT) to properly deal with the electronic structures of these apparently simple main‐group systems remains a contributing factor. [ABSTRACT FROM AUTHOR]

The halogen bond complexes CF 3 X⋯Y and C 2 F 3 X⋯Y, with Y = furan, thiophene, selenophene and X = Cl, Br, I, have been studied by using DFT and CCSD(T) in order to understand which factors govern the interaction between the halogen atom X and the aromatic ring. We found that PBE0-dDsC/QZ4P gives an adequate description of the interaction energies in these complexes, compared to CCSD(T) and experimental results. The interaction between the halogen atom X and the π -bonds in perpendicular orientation is stronger than the interaction with the in-plane lone pairs of the heteroatom of the aromatic cycle. The strength of the interaction follows the trend Cl < Br < I; the chalcogenide in the aromatic ring nor the hybridization of the C–X bond play a decisive role. The energy decomposition analysis shows that the interaction energy is dominated by all three contributions, viz., the electrostatic, orbital, and dispersion interactions: not one factor dominates the interaction energy. The aromaticity of the ring is undisturbed upon halogen bond formation: the π -ring current remains equally strong and diatropic in the complex as it is for the free aromatic ring. However, the spin-orbit coupling between the singlet and triplet π → π * states is increased upon halogen bond formation and a faster intersystem crossing between these states is therefore expected. [ABSTRACT FROM AUTHOR]

The formal Cu(III) complex [Cu(CF3)4]1− has often served as a paradigmatic example of challenging oxidation state assignment – with many reports proposing conflicting descriptions. Here we report a computational analysis of this compound, employing Energy Decomposition Analysis and Intrinsic Bond Orbital Analysis. We present a quasi‐d10 perspective of the metal centre, resulting from ambiguities in d‐electron counting. The implications for describing reactions which undergo oxidation state changes, such as the formal reductive elimination from the analogous [Cu(CF3)3(CH2Ph)]1− complex (Paeth et al. J. Am. Chem. Soc. 2019, 141, 3153), are probed. Electron flow analysis finds that the changes in electronic structure may be understood as a quasi‐d10 to d10 transition at the metal centre, rendering this process essentially redox neutral. This is reminiscent of a previously studied formal Ni(IV) complex (Steen et al. Angew. Chem. Int. Ed. 2019, 58, 13133–13139), and indicates that our description of electronic structure has implications for the understanding of elementary organometallic reaction steps. [ABSTRACT FROM AUTHOR]

Extensive tests have been performed to benchmark and to compare with second-order perturbation theory based on a complete active space self-consistent field reference function (CASPT2), the recently developed n-electron valence state perturbation theory at second order (NEVPT2). Test calculations included the group fifteen diatomic molecules X[sub 2] (X=N, P, As, and Sb) and the [sup 4]S/[sup 2]D and [sup 4]S/[sup 2]P splittings for the corresponding atoms, the [sup 1]A[sub 1]–[sup 3]B[sub 1] splittings for CH[sub 2] and SiH[sub 2], and the absorption spectra of pyrrole and of Cu(Imidazole)[sub 2](SH)(SH[sub 2])[sup +], which is a model for plastocyanin. Comparisons with full configuration-interaction calculations and experimental data show that the accuracy of NEVPT2 is in most cases even better than CASPT2. Where intruder states hamper the CASPT2 calculations, NEVPT2 performs significantly better. Care is needed in the choice of active orbitals, for example in the calculation of the [sup 4]S/[sup 2]D and [sup 4]S/[sup 2]P splittings for the group fifteen atoms. This is due to the different treatment of orbitals belonging to the inactive or active spaces, making the NEVPT2 not invariant for the choice of active space, even in cases where the multiconfiguration self-consistent field energy is invariant. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: A recent report on the crystal structure of the pentagonal‐pyramidal hexamethylbenzene dication C6(CH3)62+ by Malischewski and Seppelt [Angew. Chem. Int. Ed. 2017, 56, 368] confirmed the structural proposal made in the first report of this compound in 1973 by Hogeveen and Kwant [Tetrahedron Lett. 1973, 14, 1665]. The widespread attention that this compound quickly gained led us to reinvestigate its electronic structure. On the basis of intrinsic bond orbital analysis, effective oxidation state analysis, ring current analysis, and comparison with well‐established coordination complexes, it is demonstrated that the central carbon atom behaves like a transition metal. The central (apical) carbon atom, although best described as a highly Lewis‐acidic carbon atom coordinated with an anionic cyclopentadienyl ligand, is also capable of acting as an electron‐pair donor to a formal CH3+ group. The different roles of coordination chemistry are discussed. [ABSTRACT FROM AUTHOR]

In this study, we benchmark density functional theory gauge-including projector-augmented-wave (GIPAW) chemical shieldings against molecular shieldings for which basis set completeness has been achieved [Jensen et al., Phys. Chem. Chem. Phys. 18, 21145 (2016)]. We demonstrate the importance of two-center corrections for GIPAW hydrogen shieldings. For the other nuclei studied, standard GIPAW is sufficiently accurate. We find that GIPAW can be pushed to closely approach the basis set limit. The only source of small inaccuracies lies in the contribution to the shielding that is caused by surface currents, which we estimate comparing GIPAW susceptibilities to converged molecular magnetizabilities. [ABSTRACT FROM AUTHOR]

The use of larger basis sets to approach the complete basis limit, now common in quantum chemistry, is applied for the first time to a range of valence bond functions for the simplest case of molecular hydrogen. Good convergence of the energy is slow due to difficulty in getting a correct cusp near the nuclei, but it is significant. The form of the orbitals converges much faster, leading to a slight distortion of the valence bond orbitals and an enhanced overlap, irrespective whether the basis set is restricted to basis functions centred on one atom for each valence bond orbital or the full use of the basis set is allowed. This blurs the distinction between these two approaches and shows that basis set restrictions are not tenable in the complete basis set limit. Furthermore, it supports the general use of the full basis as advocated in the spin-coupled and generalised valence bond methods. [ABSTRACT FROM AUTHOR]

In contrast to the equilibrium structure of the homoaromatic Cs homotropenylium cation, C8H9+ (1), which supports a pinched diatropic ring current, the C2v transition state (2) for inversion of the methylene bridge of 1 is antiaromatic and supports a two-lobe paratropic π current, as detected by plotting B3LYP/6-31G** ipsocentric current maps. Participation of the bridge CH bonds is crucial for the change in global character of the current in the transition state, as shown by the quenching of its paratropicity on substitution of H by F. Orbital-based arguments allow rationalization of this transition between homoaromaticity and hyper(conjugative) antiaromaticity. More generally, the hyperconjugative ring current in a family of C2v planar-constrained geometries of (CR2)CN-1HN-1q homoannulenes (R = H, F) can be switched from paratropic (antiaromatic) to diatropic (aromatic) by variation of ring size, charge, and bridge substituent. An orbital-based counting rule accounts for these systematic trends. [ABSTRACT FROM AUTHOR]

The spin crossover behavior of the two [Fe(mtz)6]2+ complexes occupying different lattice sites in Fe(mtz)6(BF4)2 is addressed by combining quantum chemical calculations with a careful analysis of the crystal structure. It is first established from the calculations that the energy difference between high spin and low spin states depends on the orientation of the tetrazole ligands; small rotation angles favor the low spin state, while for angles larger than ∼20° the high spin state is more stable. The crystal structure shows that the two complexes have different average rotation angles of the ligands. It is larger for the site that remains HS down to low temperatures and smaller for the site that shows spin crossover to LS. The origin of the different rotation angles is found to be determined by a subtle interplay amongst steric repulsion between the ligands, H…F interactions between the complex and the counterions, and intersite interactions involving N…H contacts and π–π interactions between the N=N double bonds of the tetrazole rings. [ABSTRACT FROM AUTHOR]

The B202- cluster is predicted to exhibit a planar sheet-like structure with a circular circumference. Orbital plots and energy correlations demonstrate the close correspondence between the electronic structure of B202- and the Bessel functions describing the waves of a quantum mechanical particle confined to a disk. The π-band of B202-, and its B19- congener, contains 12 π-electrons, forming a (1σ)2(1π)4(1δ)4(2σ)2 configuration, which corresponds to a "disk aromaticity" electron count. The analogy not only applies to the π-band, but also extends to the 50 valence σ-electrons. The occupied δ-orbitals are assigned on the basis of radial and angular nodes of the scalar disk waves. The magnetic response of the cluster was examined by Nucleus Independent Chemical Shift (NICS) values and current density calculations based on the ipsocentric model. B202- is found to exhibit a remarkable inner paratropic current in the σ-channel and an outer diatropic current in the π-channel. The orbital excitations responsible for the antiaromaticity in σ and the disk-aromaticity in π are identified. [ABSTRACT FROM AUTHOR]

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling–Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations. [ABSTRACT FROM AUTHOR]

The N,C,N'-bonded arylrutheriium 2,2':6',2"-terpyridine (tpy) complex salts [Au(NCN)(tpy)](Cl) ([1a](Cl), NCN = 2,6-bis[(dimethylamino)methyl]phenyl) and [Ru(NCN)(tpy)](PF6), ([2a](PF6), NN = 2,6-bis(2-pyridyl)phenyl) can be halogenated under very mild conditions by oxidation with copper(ll) halogen salts. Halogenation occurs exclusively para to the site of metalation and yields the cations [Ru(4-R- NCN)(tpy)]+ (R= Cl, [1b]+ and R= Br [1c]+) and [Ru(4-R-NCN)(tpy)]+ (R = Cl, [2b]+ and R = Br[2c]+). In the presence of an excess of oxidant relative to [1a]+, the halogenation reaction follows first order kinetics in the oxidized ruthenium complex. However, by using a small excess of copper(ll) compared to [1a]+, dimerization of the complex cation to [(Ru(tpy))2(μ-NCN-NCN)]4+ ([3]4+) is observed, which obeys second order kinetics. Both halogenation (C-X coupling) and dimerization (C-C coupling) are a result of the unique properties of the ruthenium(lll) complexes compared to their parent ruthenium(ll) species. According to the nature of the highest occupied spin orbital (HOSO) in DFT calculations the unpaired electron in [1a]2+ and [2a]2+ is partially localized on the para position. The involvement of the cyclometalated ligand in the HOSO is supported by redox data and electronic absorption spectroscopy. The ruthenium(lll) species can best be considered a persistent organometallic radical. [ABSTRACT FROM AUTHOR]

Ring-current maps give an immediate visualisation of aromaticity on the magnetic criterion-by which a cyclic system that supports diatropic (paratropic) current induced by a perpendicular magnetic field is aromatic (anti-aromatic). Calculations of maps with the ipsocentric choice of origin are made in the 6-31G** basis set at Hartree-Fock (HF) and density functional (DFT) levels (PW91 and B3LYP functionals) on porphyrin, porphycene, orangarin, sapphyrin and hexabenzocoronene. In these systems, DFT and HF approaches produce optimal geometries with different point-group symmetries and/or different patterns of bond alternation. The ring-current maps derived with all four combinations of methods indicate that the main features of the current (global nature, direction, estimated strength) survive in systems with symmetry-breaking, but that choice of geometry is more critical for the detail of the current than is the electronic-structure method. [ABSTRACT FROM AUTHOR]

To investigate the consequences of cyclometalation for electronic communication in dinuclear ruthenium complexes, a series 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridged diruthenium complexes was prepared and studied. These complexes have a central tppz ligand bridging via nitrogen-to-ruthenium coordination bonds, while each ruthenium atom also binds either a monoanionic, N,C,N'-tetradenate 2,6-bis(2'-pyridyl)pheryl (R-N C N) ligand or a 2,2':6',2"-terpyridine (tpy) ligand. The N,C,N"-, that is, biscyclometalation, instead of the latter N,N',N"-bonding motif significantly changes the electronic properties of the resulting complexes. Starting from well-known [(Ru(tpy))[sub2](μ-tppz)][sup4+] (tpy=2,2':2'',6-terpyridine) ([3][sup4+]) as a model compound, the complexes [(Ru(R-N C N))(μ-tppz)(Ru(tpy))][sup3+] (R-N C(H) N = 4-R-1,3-dipyridylbenzene, R=H([4a][sup3+]). CO[sub2]Me (4b][sup3+])), and [(Ru(R-N C N))[sub2](μ-tppz)][sup2+], (R=H([5a][sup2+]), CO[sub2]Me ([5b][sup2+])) were prepared with one or two N,C,N'-cyclometalated terminal ligands. The oxidation and reduction potentials of cyclometalated [4][sup3+] and [5][sup2+] are shifted negatively compared to non-cyclometalated [3][sup4+], the oxidation processes being affected more significantly. Compared to [3][sup4+], the electronic spectra of [5][sup2+] display large bathochromic shifts of the main MLCT transitions in the visible spectral region with low-energy absorptions tailing down to the NIR region. One-electron oxidation of [3][sup4+] and [5][sup2+] gives rise to low-energy absorption bands. The comproportionation constants and NIR band shape correspond to delocalized Robin-Day class III compounds. Complexes [4a][sup3+] (R=H) and [4b][sup3+] (R=CO[sub2]Me) also exhibit strong electronic communication, and notwithstanding the large redox-asymmetry the visible metal-to-ligand charge-transfer absorption is assigned to originate from both metal centers. The potential of the first, ruthenium-based, reversible oxidation process is strongly negatively shifted. On the contrary, the second oxidation is irreversible and cyclometalated ligand-based. Upon one-electron oxidation, a weak and low-energy absorption arises. [ABSTRACT FROM AUTHOR]

To establish the link between the aromaticity descriptors based on the Pauling resonance energy and the molecular properties, the electric (polarizability) and magnetic (magnetizability) field response properties have been calculated using the valence bond approach for various molecules and their individual Kekulé resonance structures. The results show that there is no direct relationship between the Pauling resonance energy and the properties; the response properties are weighted averages of the properties of the individual structures. According to the aromaticity criteria based on molecular properties, one-structure benzene would be aromatic; thus, concerning molecular properties, spin-coupled bonds do not behave like localized bonds in Lewis structures, with which they are usually associated. [ABSTRACT FROM AUTHOR]

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X6H6 (X = C (1), Si (2)), X6 (X = N (3), P (4)), X3Y3H6 (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B3Y3H3 (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekul6-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekulé-like structures do not contribute. Of the heteronuclear compounds, only B3P3H6 (6) has some benzene-like features with a significant contribution of two Kekulé-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planer geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current. [ABSTRACT FROM AUTHOR]

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]-acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1–4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n + 2 and 4n π-electron conjugated circuits shows that only the 6π-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the π system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwithstanding, the relative energies of the Kekulé structures can be estimated using Randić's conjugated circuits model. [ABSTRACT FROM AUTHOR]

The 1H NMR spectrum of the piperazine-C60 monoadduct has been assigned for the first time almost 10 years after it was synthesised. The preparation and characterisation of the first (C2-substituted piperazine)-C60 monoadduct are also described, revealing the C2-substituent as occupying an exo position on the addend. [Copyright &y& Elsevier]

C(sp3)‐H and O−H bond breaking steps in the oxidation of 1,4‐cyclohexadiene and phenol by a Au(III)‐OH complex were studied computationally. The analysis reveals that for both types of bonds the initial X−H cleavage step proceeds via concerted proton coupled electron transfer (cPCET), reflecting electron transfer from the substrate directly to the Au(III) centre and proton transfer to the Au‐bound oxygen. This mechanistic picture is distinct from the analogous formal Cu(III)‐OH complexes studied by the Tolman group (J. Am. Chem. Soc. 2019, 141, 17236–17244), which proceed via hydrogen atom transfer (HAT) for C−H bonds and cPCET for O−H bonds. Hence, care should be taken when transferring concepts between Cu−OH and Au−OH species. Furthermore, the ability of Au−OH complexes to perform cPCET suggests further possibilities for one‐electron chemistry at the Au centre, for which only limited examples exist. [ABSTRACT FROM AUTHOR]

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(II)tris-bipyridine (Fe(bpy)32+), and Fe(II)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands. [ABSTRACT FROM AUTHOR]

Abstract: A well‐defined class of planar or quasi‐planar elongated boron clusters, of type B 7 + 3 n q −, serves as a basis to identify the valence bond picture of delocalized boron networks. The origin of the series is the B 7 − cluster, which exhibits σ‐aromaticity. The cluster generating step is the repetitive expansion by three boron atoms in the direction of elongation. Specific electron counting rules are obtained for π‐bonding, peripheral σ‐bonding and multicenter inner σ‐bonding. A valence bond structure is introduced which explains the remarkable regularity in the bonding pattern. The analysis supports 4c‐2e bonds as an alternative to the common 3c‐2e bonds. The results are validated by symmetry induction and ab initio calculations. [ABSTRACT FROM AUTHOR]

The zz component of the nucleus independent chemical shift or the NICS z z is commonly used as a quantifier of the (anti)aromatic character of a (sub)system. One of the underlying assumptions is that a position can be found where the “aromatic” ring currents are adequately reflected in the corresponding NICS z z value. However, as the NICS z z is the result of an integration over the entire space, it no longer explicitly contains the information needed to quantify the separate contributions arising from underlying current density patterns. In this study, we will show that these contributions can be revealed by resolving the NICS z z into orbitals, sign, and positions. Our analysis of benzene in terms of these resolutions shows that the same underlying current density can lead to highly complex shielding patterns that vary greatly depending on the position of the NICS z z‐probe. As such, our results indicate that any analysis solely based on NICS z z‐values can lead to results that are difficult to interpret, even if the system under study is considered to be well‐known. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The reported tetracoordinate dilithio methandiide complex from Liddle and co‐workers (1) is investigated from a coordination chemistry perspective, to probe the origin of its intriguing geometry. Through the application of a variety of computational techniques, non‐covalent (steric, electrostatic) interactions are found to be dominant. Further, we arrive at a bonding description which emphasizes the tricoordinate sp2‐hybridized nature of the central methandiide carbon, differing somewhat from the original proposal. Thus, 1 is distinct from other dilithio methandiides since it contains only one C−Li σ‐bond, and is found to be comparable to a simple aryllithium compound, phenyllithium. [ABSTRACT FROM AUTHOR]

A predictive model, shaped as a set of rules, is presented that predicts site‐selectivity in the mono‐oxidation of diols by palladium‐neocuproine catalysis. For this, the factors that govern this site‐selectivity within diols and between different diols have been studied both experimentally and with computation. It is shown that an electronegative substituent antiperiplanar to the C−H bond retards hydride abstraction, resulting in a lower reactivity. This explains the selective oxidation of axial hydroxy groups in vicinal cis‐diols. Furthermore, DFT calculations and competition experiments show how the reaction rate of different diols is determined by their configuration and conformational freedom. The model has been validated by the oxidation of several complex natural products, including two steroids. From a synthesis perspective, the model predicts whether a natural product comprising multiple hydroxy groups is a suitable substrate for site‐selective palladium‐catalyzed oxidation. [ABSTRACT FROM AUTHOR]

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. [ABSTRACT FROM AUTHOR]

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d‐configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non‐negligible electron‐sharing covalent character of the metal‐to‐ligand σ‐bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3)4]1−, in which the metal center tries to populate its formally unoccupied 3dx2‐y2 orbital via σ‐bonding, leading to a reduced d10 CuI description. However, Au L3‐edge X‐ray absorption spectroscopy reveals excitation into the d‐orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron‐sharing nature of the σ‐bonds in the AuI species, relative to their AgI and CuI analogues, due to the low‐lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2‐y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity. [ABSTRACT FROM AUTHOR]

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d‐configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non‐negligible electron‐sharing covalent character of the metal‐to‐ligand σ‐bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3)4]1−, in which the metal center tries to populate its formally unoccupied 3dx2‐y2 orbital via σ‐bonding, leading to a reduced d10 CuI description. However, Au L3‐edge X‐ray absorption spectroscopy reveals excitation into the d‐orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron‐sharing nature of the σ‐bonds in the AuI species, relative to their AgI and CuI analogues, due to the low‐lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2‐y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity. [ABSTRACT FROM AUTHOR]

Nucleophilic formal gold(- I) and gold(I) complexes are investigated via Intrinsic Bond Orbital analysis and Energy Decomposition Analysis, based on density functional theory calculations. The results indicate gold(0) centres engaging in electron-sharing bonding with Al- and B- based ligands. Multiconfigurational (CASSCF) calculations corroborate the findings, highlighting the gap between the electonic structures and the oxidation state formalism. [ABSTRACT FROM AUTHOR]

The ferroelectric phase transition at TC = 340 K in (C6H5CH2CH2NH3)2CuCl4 is studied by means of temperature-dependent low-frequency Raman scattering, focusing on the coupling of a low-energy librational mode to the order parameter of the transition. Analysis of the symmetry and characteristics of this mode links the dipolar order to the tilt angle of the organic cations. The thermal evolution of the Raman spectrum demonstrates the displacive component of the phase transition in combination with order-disorder phenomena and the importance of the organic-inorganic interplay to the physical properties of the compound. The ferroelectric properties investigated here can be generalized to the family of layered organic-inorganic hybrids. [ABSTRACT FROM AUTHOR]

Resonance energy is one of the criteria to measure aromaticity.The effect of the use of different orbital models is investigatedin the calculated resonance energies of cyclic conjugated hydrocarbonswithin the framework of the ab initio Valence Bond Self-ConsistentField (VBSCF) method. The VB wave function for each system was constructedusing a linear combination of the VB structures (spin functions),which closely resemble the Kekulé valence structures, and twotypes of orbitals, that is, strictly atomic (local) and delocalizedatomic (delocal) p-orbitals, were used to describe the π-system.It is found that the Pauling–Wheland’s resonance energywith nonorthogonal structures decreases, while the same with orthogonalizedstructures and the total mean resonance energy (the sum of the weightedoff-diagonal contributions in the Hamiltonian matrix of orthogonalizedstructures) increase when delocal orbitals are used as compared tolocal p-orbitals. Analysis of the interactions between the differentstructures of a system shows that the resonance in the 6π electronsconjugated circuits have the largest contributions to the resonanceenergy. The VBSCF calculations also show that the extra stabilityof phenanthrene, a kinked benzenoid, as compared to its linear counterpart,anthracene, is a consequence of the resonance in the π-systemrather than the H–H interaction in the bay region as suggestedpreviously. Finally, the empirical parameters for the resonance interactionsbetween different 4n or 4nπelectrons conjugated circuits, used in Randić’s conjugatedcircuits theory or Herdon’s semi-emprical VB approach, arequantified. These parameters have to be scaled by the structure coefficients(weights) of the contributing structures. [ABSTRACT FROM AUTHOR]

Ipsocentric calculations of current density at the B3LYP/6-311++G(2d,2p) level show that the planar Si6 ring supports a diatropic π ring current of about half the strength of the equivalent π current in benzene, both in the presumed global optimum geometry of Si6Li6 and in geometries occupying higher-energy local minima, corroborating the attribution of aromaticity to this silicon analogue of benzene. [ABSTRACT FROM AUTHOR]

Geometry optimizations have been performed for benzene and cyclobutadiene and for the corresponding moieties with nonresonating double bonds, viz. 1,3,5‐cyclohexatriene and 1,3‐cyclobutadiene. The calculations were done using the valence bond self‐consistent field method including orbital optimization. Both strictly local and delocalized p‐like orbitals were used for the π system, which influences the strengths of the π bonds. The calculations result in geometries and resonance and stabilization energies for benzene and cyclobutadiene, which are compared with theoretical models of aromaticity. The importance of resonance is discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

Crenarchaeol is a glycerol dialkyl glycerol tetraether lipid produced exclusively in Archaea of the phylum Thaumarchaeota. This membrane‐spanning lipid is undoubtedly the structurally most sophisticated of all known archaeal lipids and an iconic molecule in organic geochemistry. The 66‐membered macrocycle possesses a unique chemical structure featuring 22 mostly remote stereocenters, and a cyclohexane ring connected by a single bond to a cyclopentane ring. Herein we report the first total synthesis of the proposed structure of crenarchaeol. Comparison with natural crenarchaeol allowed us to propose a revised structure of crenarchaeol, wherein one of the 22 stereocenters is inverted. [ABSTRACT FROM AUTHOR]

Crenarchaeol is a glycerol dialkyl glycerol tetraether lipid produced exclusively in Archaea of the phylum Thaumarchaeota. This membrane‐spanning lipid is undoubtedly the structurally most sophisticated of all known archaeal lipids and an iconic molecule in organic geochemistry. The 66‐membered macrocycle possesses a unique chemical structure featuring 22 mostly remote stereocenters, and a cyclohexane ring connected by a single bond to a cyclopentane ring. Herein we report the first total synthesis of the proposed structure of crenarchaeol. Comparison with natural crenarchaeol allowed us to propose a revised structure of crenarchaeol, wherein one of the 22 stereocenters is inverted. [ABSTRACT FROM AUTHOR]

The key parameters associated to the thermally induced spin crossover process have been calculated for a series of Fe(II) complexes with mono-, bi-, and tridentate ligands. Combination of density functional theory calculations for the geometries and for normal vibrational modes, and highly correlated wave function methods for the energies, allows us to accurately compute the entropy variation associated to the spin transition and the zero-point corrected energy difference between the low- and high-spin states. From these values, the transition temperature, T1/2, is estimated for different compounds. [ABSTRACT FROM AUTHOR]

Crenarchaeol is a glycerol dialkyl glycerol tetraether lipid produced exclusively in Archaea of the phylum Thaumarchaeota. This membrane‐spanning lipid is undoubtedly the structurally most sophisticated of all known archaeal lipids and an iconic molecule in organic geochemistry. The 66‐membered macrocycle possesses a unique chemical structure featuring 22 mostly remote stereocenters, and a cyclohexane ring connected by a single bond to a cyclopentane ring. Herein we report the first total synthesis of the proposed structure of crenarchaeol. Comparison with natural crenarchaeol allowed us to propose a revised structure of crenarchaeol, wherein one of the 22 stereocenters is inverted. [ABSTRACT FROM AUTHOR]

Two finite-field implementations for the calculation of chemical shieldings of molecular systems using a plane-wave basis set and the Gauge-Including Projector-Augmented-Wave method are presented. The direct approach mimics the nuclear magnetic resonance experiment in that it puts the molecule in a uniform magnetic field and obtains shieldings from the current response. The other is based on the recently introduced 'converse method' [T. Thonhauser, D. Ceresoli, A. A. Mostofi et al., J. Chem. Phys. 131, 101101 (2009)]. In both methods two-center contributions to the shieldings can be included via a numerically simple augmentation construction. Results obtained with both methods are discussed as well as (dis)similarities in their behaviors. [ABSTRACT FROM AUTHOR]

For the search for promising singlet fission candidates, the calculation of the effective electronic coupling, which is required to estimate the singlet fission rate between the initially excited state (S0S1) and the multiexcitonic state (1TT, two triplets on neighboring molecules, coupled into a singlet), should be sufficiently reliable and fast enough to explore the configuration space. We propose here to modify the calculation of the effective electronic coupling using a nonorthogonal configuration interaction approach by: (a) using only one set of orbitals, optimized for the triplet state of the molecules, to describe all molecular electronic states, and (b) only taking the leading configurations into consideration. Furthermore, we also studied the basis set convergence of the electronic coupling, and we found, by comparison to the complete basis set limit obtained using the cc‐pVnZ series of basis sets, that both the aug‐cc‐pVDZ and 6–311++G** basis sets are a good compromise between accuracy and computational feasibility. The proposed approach enables future work on larger clusters of molecules than dimers. [ABSTRACT FROM AUTHOR]

Organic semiconductors consisting of molecules bearing polar side chains have been proposed as potential candidates to overcome the limitations of organic photovoltaics owing to their enhanced dielectric constant. However, introducing such polar molecules in photovoltaic devices has not yet resulted in higher efficiencies. A microscopic understanding of the impact of polar side chains on electronic and structural properties of organic semiconductors is paramount to rationalize their effect. Here, the impact of such side chains on bulk heterojunction overall morphology, molecular configurations at donor–acceptor (DA) interfaces, and charge carrier energy levels is investigated. The multiscale modeling approach used allows to resolve DA interfaces with atomistic resolution while taking into account the large‐scale self‐organization process which takes place during the processing of an organic thin film. The polar fullerene‐based blends are compared to the well‐studied reference system, poly(3‐hexyl‐thiophene) (P3HT):phenyl‐C61‐butyric acid methyl ester (PCBM). Introduction of polar side chains on a similar molecular scaffold does not affect molecular orientations at the DA interfaces; such orientations are, however, found to be affected by processing conditions and polymer molecular weight. Polar side chains, instead, are found to impact considerably the charge carrier energy levels of the organic blend, causing electrostatic‐induced broadening of these levels. [ABSTRACT FROM AUTHOR]

The good performance of N‐heterocyclic carbenes (NHCs), in terms of versatility and selectivity, has called the attention of experimentalists and theoreticians attempting to understand their electronic properties. Analyses of the Au(I)–C bond in [(NHC)AuL]+/0 (L stands for a neutral or negatively charged ligand), through the Dewar–Chatt–Duncanson model and the charge displacement function, have revealed that NHC is not purely a σ‐donor but may have a significant π‐acceptor character. It turns out, however, that only the σ‐donation bonding component strongly correlates with one specific component of the chemical shielding tensor. Here, in extension to earlier works, a current density analysis, based on the continuous transformation of the current density diamagnetic zero approach, along a series of [(NHC)AuL]+/0 complexes is presented. The shielding tensor is decomposed into orbital contributions using symmetry considerations together with a spectral analysis in terms of occupied to virtual orbital transitions. Analysis of the orbital transitions shows that the induced current density is largely influenced by rotational transitions. The orbital decomposition of the shielding tensor leads to a deeper understanding of the ligand effect on the magnetic response properties and the electronic structure of (NHC)‐Au fragments. Such an orbital decomposition scheme may be extended to other magnetic properties and/or substrate‐metal complexes. [ABSTRACT FROM AUTHOR]

A comparative study on the self‐assembly of sexiphenyl‐dicarbonitrile on highly oriented pyrolytic graphite and single‐layer graphene on Cu(111) is presented. Despite an overall low molecule–substrate interaction, the close‐packed structures exhibit a peculiar shift repeating every four to five molecules. This shift has hitherto not been reported for similar systems and is hence a unique feature induced by the graphitic substrates. Stepping on the surface: The self‐assembly of sexiphenyl‐dicarbonitrile on highly oriented pyrolytic graphite and graphene on Cu(111) was studied using scanning tunneling microscopy. The close‐packed networks exhibit a peculiar shift every four or five molecules (see figure). This shift is a unique feature induced by the graphitic substrates. [ABSTRACT FROM AUTHOR]

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn*Sham method to calculate the electronic contribution to the dielectric constant for fullerene C60 derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C60. [ABSTRACT FROM AUTHOR]

Control over the morphology of the active layer of bulk heterojunction (BHJ) organic solar cells is paramount to achieve high-efficiency devices. However, no method currently available can predict morphologies for a novel donor-acceptor blend. An approach which allows reaching relevant length scales, retaining chemical specificity, and mimicking experimental fabrication conditions, and which is suited for high-throughput schemes has been proven challenging to find. Here, we propose a method to generate atom-resolved morphologies of BHJs which conforms to these requirements. Coarse-grain (CG) molecular dynamics simulations are employed to simulate the large-scale morphological organization during solution-processing. The use of CG models which retain chemical specificity translates into a direct path to the rational design of donor and acceptor compounds which differ only slightly in chemical nature. Finally, the direct retrieval of fully atomistic detail is possible through backmapping, opening the way for improved quantum mechanical calculations addressing the charge separation mechanism. The method is illustrated for the poly(3-hexyl-thiophene) (P3HT)-phenyl-C61-butyric acid methyl ester (PCBM) mixture, and found to predict morphologies in agreement with experimental data. The effect of drying rate, P3HT molecular weight, and thermal annealing are investigated extensively, resulting in trends mimicking experimental findings. The proposed methodology can help reduce the parameter space which has to be explored before obtaining optimal morphologies not only for BHJ solar cells but also for any other solution-processed soft matter device. [ABSTRACT FROM AUTHOR]

The Dewar-Chatt-Duncanson (DCD) model provides a successful theoretical framework to describe the nature of the chemical bond in transition-metal compounds and is especially useful in structural chemistry and catalysis. However, how to actually measure its constituents (substrate-to-metal donation and metal-to-substrate back-donation) is yet uncertain. Recently, we demonstrated that the DCD components can be neatly disentangled and the π back-donation component put in strict correlation with some experimental observables. In the present work we make a further crucial step forward, showing that, in a large set of charged and neutral N-heterocyclic carbene complexes of gold(I), a specific component of the NMR chemical shift tensor of the carbenic carbon provides a selective measure of the σ donation. This work opens the possibility of 1) to characterize unambiguously the electronic structure of a metal fragment (LAu(I) n+/0 in this case) by actually measuring its σ-withdrawing ability, 2) to quickly establish a comparative trend for the ligand trans effect, and 3) to achieve a more rigorous control of the ligand electronic effect, which is a key aspect for the design of new catalysts and metal complexes. [ABSTRACT FROM AUTHOR]

A multidisciplinary approach involving organic synthesis and theoretical chemistry was applied to investigate a promising strategy to improve charge separation in organic photovoltaics: installing permanent dipoles in fullerene derivatives. First, a PCBM analogue with a permanent dipole in the side chain (PCBDN) and its reference analogue without a permanent dipole (PCBBz) were successfully synthesized and characterized. Second, a multiscale modeling approach was applied to investigate if a PCBDN environment around a central donor-acceptor complex indeed facilitates charge separation. Alignment of the embedding dipoles in response to charges present on the central donor-acceptor complex enhances charge separation. The good correspondence between experimentally and theoretically determined electronic and optical properties of PCBDN, PCBBz, and PCBM indicates that the theoretical analysis of the embedding effects of these molecules gives a reliable expectation for their influence on the charge separation process at a microscopic scale in a real device. This work suggests the following strategies to improve charge separation in organic photovoltaics: installing permanent dipoles in PCBM analogues and tuning the concentration of these molecules in an organic donor/acceptor blend. [ABSTRACT FROM AUTHOR]

The tetrapyridyl ligand bbpya (bbpya=N,Nbis( 2,2'-bipyrid-6-yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)2] (1) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai's domain model, and powder X-ray diffraction measurements, 1 is low-spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N–H···S intermolecular H-bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1. DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)( NCS)2] (2). Periodic DFT calculations suggest that crystal-packing effects are significant for compound 2, in which they destabilize the HS state by about 1500 cm–1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing. [ABSTRACT FROM AUTHOR]

In this paper we investigate the acoustic-to-optical up-conversion phonon processes in a multicomponent system. These processes take place during heat transport and limit the efficiency of heat flow. By combining time-resolved optical and heat capacity experiments we quantify the thermal coupling constant to be g ~ 0.4 1017 W/Km3. The method is based on selective excitation of a part of a multicomponent system, and the measurement of the thermalization dynamics by probing the linear birefringence of the sample with femtosecond resolution. In particular, we study a layered multiferroic organic-inorganic hybrid, in the vicinity of the ferroelectric phase transition. A diverging term of the heat capacity is associated to soft-mode dynamics, in agreement with previous spectroscopy measurements. [ABSTRACT FROM AUTHOR]

Current organic semiconductors for organic photovoltaics (OPV) have relative dielectric constants (relative permittivities, εr) in the range of 2-4. As a consequence, Coulombically bound electron-hole pairs (excitons) are produced upon absorption of light, giving rise to limited power conversion efficiencies. We introduce a strategy to enhance εr of well-known donors and acceptors without breaking conjugation, degrading charge carrier mobility or altering the transport gap. The ability of ethylene glycol (EG) repeating units to rapidly reorient their dipoles with the charge redistributions in the environment was proven via density functional theory (DFT) calculations. Fullerene derivatives functionalized with triethylene glycol side chains were studied for the enhancement of εr together with poly( p-phenylene vinylene) and diketopyrrolopyrrole based polymers functionalized with similar side chains. The polymers showed a doubling of εr with respect to their reference polymers in identical backbone. Fullerene derivatives presented enhancements up to 6 compared with phenyl-C61-butyric acid methyl ester (PCBM) as the reference. Importantly, the applied modifications did not affect the mobility of electrons and holes and provided excellent solubility in common organic solvents. [ABSTRACT FROM AUTHOR]